The process of separating hydrocarbon gas constituents is well known in the art. If the process utilizes a demethanizer then methane and other more volatile components are separated from the ethane and less volatile components of the gas stream. Should, however, the process utilize a de-ethanizer, then ethane and more volatile components (including methane) are separated from propane and less volatile components of the stream. In either case, the "recovered" components are in the column bottoms product while the "rejected" components are in the overhead vapor stream.
Many patents relating to this technology exist with each describing a unique method or improvement. Their emphasis, however, is generally directed to the processing of the hydrocarbon gas before it enters a demethanizer, de-ethanizer of other similar structure. Seldom is there emphasis on the processing of the overhead vapor stream after it leaves the demethanizer, etc. (e.g., U.S. Pat. Nos. 4,453,958 and 4,464,190 to Gulsby). However, some of those that do process the overhead vapor stream, such as U.S. Pat. Nos. 4,171,964 and 4,278,457 to Campbell et al., simply warm this stream from the demethanizer in a heat exchanger before transporting it elsewhere.
Other patents, such as U.S. Pat. Nos. 4,318,723 and 4,350,511 to Holmes et al., not only warm this overhead vapor stream, but also condense a portion of it and return this condensed portion to the distillation column as reflux. Perhaps the most pertinent patent is U.S. Pat. No. 4,687,499 to Aghili which first warms then compresses the overhead vapor stream before returning a portion of it back to the demethanizer as reflux, but only after this reflux portion has first been chilled and expanded.
While all of these processes are functional, only Holmes et al. and Aghili recycle a portion of the removed vapor stream back to the column as a liquid. Holmes et al. condenses a portion of the vapor stream while Aghili's expansion of the chilled overhead vapor product causes some of this gas to condense into a liquid. The return of a liquid reflux is desirable because it is the condensed liquid that increases the recovery percentage of the desired column bottoms product. Analysis has shown that the reflux effect is optimized when the vapor recycle stream is totally condensed before expansion to the demethanizer operating pressure. Subcooling of the condensed reflux stream is usually less effective than increasing the rate of non-subcooled condensed liquid. Unfortunately, however, a large portion of the reflux streams of Holmes et al. and Aghili are still vapor which does not increase product recovery. Instead, this uncondensed vapor mixes with the residue gas in the demethanizer and both are summarily discharged as the overhead vapor stream.
It is thus an object of this invention to provide a process whereby a recycle stream containing liquid is returned to the top of a demethanizer for increased ethane recovery in the column bottoms product. An additional object of this invention is to provide a recycle stream that is totally condensed thereby maximizing the recovery of ethane. A further object is to reduce the size of the recycle stream equipment and hardware yet provide the same amount of liquid reflux to the top of the demethanizer as is currently accomplished by Aghili or Holmes et al. Another object, should the same size equipment be used for this process as is used for conventional processes, it to deliver more liquid reflux to the top of the demethanizer column. Still another object of this invention is to reduce the number of expander-compressors and other equipment needed since the recycle stream is fully condensed. These and other objects of this invention will become apparent upon a further reading of this application.